This invention relates to longitudinal mode laser diodes and, more particularly, to achieving higher yields of devises lasing at a desired wavelength.
Existing semiconductor laser fabrication processes have difficulty in achieving devices that oscillate at a precise wavelength. One of the contributing factors is a variation of the material gain function that may arise during epitaxial crystal growth. Typical variation in lasing wavelength of lasers made from different growths may amount to +/xe2x88x92 5 nm. However many applications require lasers to have a wavelength accuracy of +/xe2x88x92 1 nm or better, e.g., for the optical pumping of Nd:YAG lasers at 808 nm. Other factors such as stress, temperature and thermal non-uniformities often cause additional variable shifts. The mismatch between the achievable wavelength variation and end-user specifications has led to difficulties in achieving desirable manufacturing yields.
The most common type of laser diode structure is the single quantum well which uses a ternary or quaternary material such as AlGaAs or GalnAsP in which a narrow bandgap, optically active, thin layer (xcx9c0.05 xcexcm thick) is sandwiched between a pair of thicker, wider-bandgap waveguide and cladding layers. A semiconductor, edge-emitting (longitudinal mode) diode laser typically includes a resonator formed of a solid state laser gain medium extending longitudinally between input and output mirror surfaces usually formed by cleaving. The distance between the facets defines a F-P (F-P) cavity which is capable of sustaining several different longitudinal lasing modes. The gain medium is typically enclosed on laterally adjacent sides by reflective material having an index of refraction n2 which is greater than the index of refraction n1 of the gain medium material.
To improve wavelength stability, feedback can be provided by locating a first or second-order grating of suitable pitch either within the gain region of the solid state structure (for the DFB laser), or externally thereto (for the DBR laser). For example, U.S. Pat. No. 4,178,604 issued Dec. 11, 1979 shows a laser diode operating at 0.875 xcexcm stabilized by a first order grating having a pitch xcex9=0.123 xcexcm and an index of refraction n≅3.55 located between the active layer (n=3.6) and another layer (n=3.36). The grating is etched in a process using ultraviolet light to holographically etch a semiconductor layer located 0.3 xcexcm away from the 0.1 xcexcm thick active layer and having a refractive index lower than that of the active layer using interfering ultra-violet beams. The grating can be termed a xe2x80x9cfirst orderxe2x80x9d grating since the pitch xcex9=0.123 xcexcm is related to the desired laser wavelength xcex=0.875 xcexcm by xcex9=xcex/2n. Unfortunately, ultraviolet light holography requires extensive measure to prevent unwanted carbon coating of optics due to photolyzation of organic vapors in the air which generally dictates that the process be performed in an extensively purged environment. Moreover, the surface of the mirror used to reflect the interfering waves must be extremely smooth to avoid unwanted scattering that would detract from the precise exposure of the photoresist, scattering being proportional to the inverse-fourth power of the wavelength of light employed. Finally, UV lasers are notoriously unreliable. It would be extremely desirable to obtain a manufacturing process that did not require the use of lasers that produce UV light.
Another example of a DFB laser appears in vol. 18 Electronics Letters for Jan. 7, 1982, at pp. 27, 28 which shows a GaInAsP/InP laser operating at a wavelength of 1.5 xcexcm using a second order grating (xcex9=0.4522xcexc) etched into an n-InP substrate. The grating was buried in a heterostructure comprised of a 0.17 xcexcm thick waveguide layer of Sn-doped, n-GaInAsP adjacent to a non-doped, 0.19 xcexcm thick GaInAsP active layer. To suppress unwanted F-P, modes the rear facet of the cavity was inclined.
U.S. Pat. No. 4,704,720 issued Nov. 3, 1987 asserted that the grating used in the aforementioned laser was located too far away from (i.e., too weakly coupled to) the optical field so that, at certain values of the injected current, oscillation at the unwanted Fabry-Perot modes occurred instead of at the desired single wavelength. Accordingly, the ""720 patent laser, operating at an exemplary wavelength of 1.3 xcexcm, located its second-order grating (having a pitch of xcex9=0.4xcexc) in the strongest part of the optical field to obtain oscillation in a single longitudinal mode. Alternatively, a first order grating having a finer pitch (xcex9=0.2xcexc) was suggested.
While a second-order grating can be produced using lower energy blue light beams in the photolithographic process, second-order gratings located in the high intensity optical field of the active layer or of the waveguide layer give rise to diffraction orders that sap energy from the single desired longitudinal mode. It would, however, facilitate ease of manufacture if blue light beams could be used to produce second-order gratings on wafers made from production runs of crystal growths having reasonable variation in their material gain function without incurring the penalty of energy loss through surface-diffraction losses. Wavelength stability is especially desirable at 0.808 xcexcm which is the wavelength required to optically pump Nd:YAG lasers. One conventional arrangement has 19 laser gain stripes per bar so that it can be coupled to a bundle of 19 optical fibers, the output of which can be conveniently coupled into the Nd:YAG laser rod. It would be desirable to obtain a laser bar having sufficient wavelength stability among a comparable number of diodes on the bar to drive the fiber bundle.
In the copending application entitled xe2x80x9cIncreasing The Yield Of Precise Wavelength Lasersxe2x80x9d, Ser. No. 09/848,529, filed May 3, 2001, there is disclosed an arrangement for overcoming the material gain function of the semiconductor material of the wafer that would tend to lase at an unwanted wavelength, xcex2. A second-order dielectric grating was embedded between epitaxial, gain-providing layers having different indices of refraction. The layer in which the grating was etched was located at a distance sufficiently remote from the high intensity optical field of the waveguide to provide just enough feedback to reduce the gain at the unwanted xcex2 wavelength and yet support oscillation at a desired wavelength xcex1 without incurring excessive surface diffraction loss. In the aforementioned patent application, feedback from the grating effected a gain discrimination factor, xcex1 having an order of magnitude of 0.1 cmxe2x88x921. The embedded grating provided stabilizing feedback and reduced the fraction of power lost to surface diffraction to less than 1 percent. When the wafer was processed into F-P laser devices, the coated facets provided most of the feedback, while the feedback from the grating altered the F-P longitudinal mode spectrum to create a preferred resonance condition.
While the aforementioned copending application provided a way to increase the yield of lasers having a precise wavelength, difficulties were encountered in growing the necessary epitaxial gain-providing layers over the etched grating layer. As is well known, growing epitaxial semiconductor layers that will exhibit the characteristics of a single crystal requires that extreme care be taken with respect to the surface on which the layers are to be grown. Unfortunately, etching processes tend to leave behind various oxides and impurities as well as surface defects that prevent the formation of single crystal growth structures on a disturbed surface. Moreover, the direction in which the grating lines are etched can also contribute to difficulties in obtaining overgrowth with precise characteristics.
To achieve the wavelength stability described in the aforementioned application, the grating lines were etched in the direction parallel to the emitting facet, i.e. in the xe2x80x9c0{overscore (11)}xe2x80x9d, direction, throughout the longitudinal dimension of the laser. Depending on the etchant and processing techniques employed, etching the grating lines in xe2x80x9c0{overscore (11)}xe2x80x9d direction, produces xe2x80x9cdovetailxe2x80x9d grooves rather than xe2x80x9cvxe2x80x9d grooves. Difficulties have been experienced in growing semiconductor layers having precisely defined semiconductor overgrowth over a layer having a grating defined by dovetail grooves.
While it would be possible to orient the wafer so that the grating lines could be etched in the v-groove direction, this would entail disruption of the usual process in which a bar containing multiple laser emitters is formed from a semiconductor wafer. In such a process, v-grooves need to be etched between the lasers on the wafer (i.e., perpendicular to the laser facets) in order to prevent parasitic lasing along the xe2x80x9cwidthxe2x80x9d or transverse direction of the bar. If the v-grooves are to be etched perpendicular to the facets, then the gratings etched parallel to the facet will be dovetail grooves. Accordingly, one who would fabricate wavelength stable lasers is faced with the dilemma of either changing the laser bar fabrication process or of etching the grating lines in a direction that impedes proper epitaxial growth of semiconductor layers over the grating layer.
In accordance with the principles of the present invention, a conventional process of fabricating laser bars from a wafer may be employed in which the diodes of the bar are separated from each other by v-grooves etched perpendicular to the facets to prevent lasing in the direction transverse of the bars. A grating will be etched into a cladding layer for a short distance beyond the end of the gain stripe of each diode on the bar. Having a length advantageously only about ten percent of the distance between the facets, or shorter, and lying beyond the end of the gain stripe, the grating is located in an unpumped region of the laser. The grating so located may advantageously be etched using a process that results in xe2x80x9cdovetailxe2x80x9d etching since the grating need not be overgrown with gain-providing epitaxial layers. The short grating is located at an appropriate distance away from the waveguide region, beyond either or both ends of the gain stripe, but preferably near the highly reflective (rear) facet of the F-P cavity. The grating so located will scatter less optical energy because of the lower optical intensity existing at the rear facet than at the low reflectivity (front) facet. Since the grating length is one-tenth the length of the embedded grating used in the aforementioned copending application, the feedback gain discrimination factor, xcex1, should be ten times larger, e.g., xcex1≈1.0 cmxe2x88x921 than when an embedded, full-length grating is employed. This will hold the fraction of power lost to surface diffraction radiation to about 1%, which is sufficient to provide stabilizing feedback without sapping too much energy from the longitudinal beam.